1. Field of the Invention
The full invention consists of an air purging system in combination with a compliant storage chamber having at least one pliable wall that forms a venous reservoir and a two-chamber cardiotomy reservoir incorporated atop the storage chamber. The combined units provide a collapsible “closed” venous reservoir unitized with cardiotomy reservoir, with vented blood separated from the sucker blood, having air removal features that improve that of a hardshell venous-cardiotomy reservoir unit. The innovative characteristics of each of the three devices support their use on their own.
2. Description of the Prior Art
During cardiopulmonary bypass, blood flow from the venous side of the patient to a venous reservoir depends on the resistance of the fluid conduit between patient and reservoir and the pressure difference between patient and reservoir. When the reservoir is maintained at atmospheric pressure, that pressure difference is the height difference between patient and reservoir; the resulting flow is referred to as gravity drainage. Venous drainage by gravity alone provides inadequate return during procedures such as minimally invasive cardiac surgery and bypass via femoral cannulation. Usually it is the resistance of the venous cannula that limits the flow rate. Vacuum assisted venous drainage (VAVD) is a technique that overcomes flow limitations by applying suction to the venous reservoir thereby increasing the pressure difference between the venous cannulation site and venous reservoir. VAVD allows for a decrease in the inner diameter (ID) of the venous line, thereby reducing prime volume and enabling the use of a smaller internal diameter cannula, which translates to an easier insertion, better surgical view and a smaller surgical incision.
Clinically, venous bags are used because they provide significant safety features: if the bag accidentally empties, it collapses, thereby preventing gross air from being pumped to the patient, they have no, or very little, air-blood interface, and they require no antifoam agents that can embolize into the blood. Designs that allow VAVD with venous bags have been previously described by Tamari's U.S. Pat. Nos. 6,337,049 and 6,773,426, and Cambron's U.S. Pat. No. 6,537,495. The present invention also allows the user the option of VAVD.
The only venous reservoir that combines a flexible wall and a rigid wall to form a “closed” variable blood chamber is that described in U.S. Pat. Nos. 4,424,190 and 4,959,062. Each of these devices has its outlet at the lowest point of the venous reservoir (see 24 in FIG. 24 of U.S. Pat. No. '062, and 36 in FIG. 3 of U.S. Pat. No. '190). This choice makes it more difficult to assure that the pliable wall seals against its mating rigid wall when the reservoir empties. The '190 patent has a drawback: once that reservoir empties, its pliable wall is sucked in at its outlet and shuts off as it should. However, when the volume returns, the flexible wall remains stuck to the outlet and does not open until the reservoir is almost full. The '062 patent's design alleviates that flaw however it introduced another problem: when the reservoir empties the rib prevents the flexible wall from completely sealing against the outlet port allowing air from the cardiotomy reservoir to be sucked in, an undesirable outcome. The present invention provides a design that allows the outlet to close completely when the reservoir empties and to easily open when less than ⅓ of the volume of a full reservoir returns.
When air enters blood, the blood foams. Hard shell reservoirs have defoamers within the venous inlet chamber (e.g. see defoamer 6 in distribution chamber 22 of FIG. 1 in Fini, U.S. Pat. No. 6,287,270). In the open, hard shell reservoir, air escapes by floating to the top of the reservoir where it is purged to atmosphere. In prior art venous bag reservoirs, air also floats to the top but it must be actively eliminated. This can be done manually with a syringe, or more frequently with a roller pump intermittently operating to remove air accumulating at the top of the bag. This is shown in FIG. 1 where foam that accumulates at the top of collapsible venous reservoir 1103 is removed by suction pump 1114 that pumps that foam/air to cardiotomy reservoir 1115. With either method, a sudden large volume of air can overwhelm the air removal system and cause disastrous consequences, especially without a vigilant perfusionist. Further, sucking blood and foam through the small diameter of the tubes and stopcocks used at the top of current bags results in blood damage. Various solutions have been proposed. Tamari's U.S. Pat. Nos. '049 and '426 eliminate air from a venous bag automatically by utilizing a floating ball valve, a hydrophobic membrane, or a level sensor. All three systems require suction to remove the air. Also, with current available membrane materials, the membrane has limited effective life.
Tamari's '049 and '426 patents also describe a blood chamber with a level sensor that activates a vacuum source to remove the air when the blood level in the chamber drops below that sensor. The design provides automated means to remove air from that chamber and that chamber in combination with a blood chamber having at least one flexible wall allows automated air removal and VAVD with a venous bag but does not, as the present invention, allow for passive air removal.
Active air removal required for venous bags also results in more difficult priming. Thus, once a bag is primed and the prime solution is recirculated, the air in the rest of the circuit is returned to the bag, again filling it with air. A better system would not allow air to return to the primed bag. This would reduce priming time and the possibility that air can remain in the bag.
State-of-the-Art soft-shell venous reservoirs with a screen are poorly designed: a large portion of their screen contacts the internal walls of the bag rendering that portion of the screen ineffective and thereby increasing the resistance to blood flow across the screen. This is illustrated in FIG. 1a, a line drawing of a prior art bag made by Cobe, FIG. 1aa, a line drawing of a cross section along line 1aa-1aa′ of the bag shown in FIG. 1a, and FIG. 1ab, an enlargement of the circled section of the bag shown in FIG. 1aa. As illustrated, at low volume most of the screen contacts the walls of the bag and is unavailable for blood flow. The contact area of the screen contacting the walls of the bag increases as the volume in the reservoir decreases. Two typical prior art venous bags exhibiting these problems are the ones described in Hess's U.S. Pat. No. 5,573,526 and in Stewart's U.S. Pat. No. 5,720,741. The latter is depicted in the present application in FIG. 1a as PRIOR ART. One aspect of the present invention eliminates contact between the screen and the walls of the chamber housing it, thereby allowing the use of a screen with smaller area and/or smaller pores. Though Tamari's '049 and '426 patents illustrate designs that prevent the screen from contacting the wall of the reservoir, (e.g. FIGS. 7a and 7b of '049), those designs in combination with a venous reservoir require that controlled vacuum be applied to the top of the venous bag to remove air.
As the pore size of filter screen used to trap bubbles decreases so does the effective open area and resistance to flow. For example, the preferred screen used for State-of-the-Art venous bag has a pore size of 105μ and an open area of 52% as compared to an open area of only 30% for a screen with a pore size of 37μ. However, as described in reference to screen 2c in FIG. 3a, the use of smaller pore screen reduces the size and number of bubbles. The larger effective screen area of the present invention allows using a smaller pore size screen and still maintaining a total open area that is equal to, or is even larger than, State-of-the-Art bags thereby maintaining a blood velocity across the screen that equals to or is lower than that associated with current bags. A lower velocity translates to lower number of bubbles crossing the screen.
At low blood levels, or if there is air in current soft shell reservoirs, the indication of blood level is inaccurate and the alarms, mostly designed for hard shell reservoirs, are not reliable. The present invention provides more reliable means to alarm at low blood levels.
A softshell reservoir with integrated cardiotomy reservoir is described in Elgas's U.S. Pat. No. 5,935,093. That softshell blood reservoir incorporates an integral flexible cardiotomy section in which a filter/defoamer unit is supported in a semirigid cage. The reservoir also incorporates a storage section and a mixing section. The three sections can selectively communicate with each other. A major shortcoming of this unit is that if the cardiotomy, in fluid communication with the venous reservoir, empties, then air can enter the venous reservoir and exit its outlet. The unit also does not allow VAVD and requires active removal of air from the venous reservoir. The present invention prevents air from the cardiotomy to exit the venous reservoir, allows VAVD and air is exhausted to atmosphere passively.
Almost all hard shell venous reservoirs incorporate a cardiotomy reservoir; the user gets a single unit (e.g. Thor et al's U.S. Pat. No. 5,411,705 and Fini's '270 patent). The lower cost, shorter setup time and ease of air removal are the major reasons that more hard shell reservoirs are used than collapsible venous reservoirs. In one form, the present invention provides the user with a collapsible venous reservoir and a cardiotomy reservoir as a single unit that is easy to setup, and is clinically safer than the State-of-the-Art unified hard shell venous-cardiotomy reservoir unit.
During CPB, “clean” blood includes blood aspirated from a venting site (e.g. the aortic root cardioplegia cannula, LV vent), blood with entrapped air withdrawn from the top of a venous bag (i.e. as used by NovoSci), and blood purged from the top of the arterial filter. Clean blood is distinctly different from the “dirty” blood sucked from the surgical field, mostly of which comes from the pericardial sack. Before clean blood is returned to the patient, its entrapped air must be removed. Vent blood may entrain a large volume of air. If that blood is added directly to venous blood, it generates a large volume of foam (and a concomitant large blood-gas area) and can easily overwhelm the active air removal systems of current minicircuits (see below). Before sucker blood can be returned it must be filtered to remove both debris (e.g. particles of fat and tissue, clots) and entrapped air. Retransfusion of cardiotomy suction blood and mediastinal shed blood increases postoperative systemic inflammatory response, hemolysis, acellular lipid deposits in the microvasculature, thrombin, neutrophil, and platelet activation, and the release of neuron-specific enolase. It is considered preferable to send this blood to a cell saver. When using venous bags both the clean and dirty blood are processed by passing through a cardiotomy reservoir, a disadvantage because the clean blood is exposed to the inlet of the filter for the dirty blood with its aforementioned deleterious consequences.
There are no cardiotomies that filter the clean blood separate from the dirty blood and keep the two apart. Some State-of-the-Art hard shell reservoirs allow clean blood to bypass the dirty blood filter by directing the clean blood into the venous blood inlet chamber. However, none provide means to purge the air out of the clean blood prior to it combining with the venous blood, thereby limiting the clinically undesirable generation of large volume of blood foam. Fini's '270 patent partially addresses this by having a valve that lets the user to either use the dirty blood (flow to the venous chamber) or to accumulate it in a chamber and then direct it to a cell saver. Yokoyama, et al.'s U.S. Pat. No. 6,908,446 illustrates a cardiotomy reservoir with a filter for the vented blood and a separate filter for dirty blood. This prevents clean blood from directly contacting foreign substances filtered off from the dirty blood but does not prevent clean blood accumulating in the cardiotomy from contacting the debris in the filter once the blood level reached the debris. The present invention addresses these drawbacks by separating the clean blood from the dirty blood as well as giving the user a choice to return the dirty blood to the bag or to a cell saver.
Blood enters the cardiotomy reservoir of prior art devices either at the top of the cardiotomy or via a tube that extends vertically downward to the bottom half of the reservoir. The former separates blood from air as it enters the reservoir but results in foam from the entering blood splashing into the blood in the reservoir. The latter, when the blood level is above the outlet of the entrance tube, results in foam from the air in the entering blood bubbling in the blood in the reservoir. The present invention allows at least the clean blood to enter along an angle that purges most of the air prior to the clean blood joining blood already in the cardiotomy while limiting splashing.
Many of the State-of-the-Art cardiotomies include a perfusion connector that accommodates a ⅜″ ID tubing, but none have a ⅜″ ID tubing extending from the top of the cardiotomy to its bottom as to allow a clotted cardiotomy filter to be bypassed using a new cardiotomy connected to the inlet of that ⅜″ tubing.
Cardiotomy reservoirs are also used to accommodate the overflow of blood volume that exceeds the capacity of the venous bag (the largest one can accommodate less than 2.0 L). Larger blood volume capacity is frequently required during aortic or mitral valve replacement during which the excess volume rises into the cardiotomy reservoir resulting in clean venous blood contacting the aggressive filter and the filtrate (e.g. fat, bone chips etc.) trapped at the bottom of the filter of the cardiotomy reservoir that can result in the aforementioned undesirable outcomes.
The blood level at which the clean blood contacts the antifoam loaded defoamer in the venous reservoir and the volume at which the blood contacts the aggressive cardiotomy reservoir filter is summarized in Table 1 for typical State-of-the-Art hard shell venous-cardiotomy reservoir units. In one form of the present invention, the volume capacity of the closed venous blood chamber and/or the “clean” chamber of the cardiotomy reservoir is higher than current systems, thereby limiting clean blood contact with the “dirty” chamber and/or defoamer while limiting blood-to-air interface (see below).
Minimizing the blood-to-air interface is a major design objective of devices used in the cardiopulmonary bypass circuit. The last two columns in Table 1 provide the surface area of air that the blood is exposed to when the reservoir contains 500 or 1000 ml of blood. As will be shown, when the venous reservoir of the present invention is filled, it has 7 to 30 times smaller blood-to-air area than State-of-the-Art hard shell venous reservoirs. The values assume that there is no air in the venous blood or foam above the blood. It should be noted that the maximum venous blood volume that can be accommodated by the State-of-the-Art units without contacting the defoamer, of either the HSVR or CR, or the dirty blood filter is 1,200 ml.
TABLE 1Nominal values for adult Prior Art hard shell reservoirs v. thepresent invention.Volume tocontactVolume toHSVR*contactBlood-Air area atModelDefoamerCR**500 ml1000 mlOne form of the3,000ml3500ml<20cm2<20cm2presentInvention***Terumo - Capiox3,000ml1,200ml78cm2240cm218SXDideco - Synthesis600ml1,400ml155cm2213cm2Dideco - Avant 903600ml3,500ml303cm2303cm2Jostra - VHK 4200300ml500ml181cm2181cm2Medtronic - Affinity1,000ml400ml232cm2232cm2CVR NTCobe - VVR4000i200ml800mlGish - CAPVRF 441200ml1200ml131cm2135cm2*HSVR—Hard shell venous reservoir,**CR—Cardiotomy reservoir,***Typical values
The defoamer of State-of-the-Art venous reservoirs is located at least along the top section of the reservoir (e.g. Terumo's SX-25) or lineup the entire screen area of the inlet section of the reservoir (Cobe's VVR 4000i). Currently it is not known whether early collapsing of blood foam that lowers blood-air interface but increases blood contact with the antifoam, is clinically better than reducing blood contact with the antifoam but increasing blood-to-air interface. One aspect of the present invention incorporates a top and bottom defoamer, wherein the bottom defoamer is smaller by volume and extends downward from the top defoamer into the inlet chamber of the air purging chamber, a geometry that provides flexibility in adjusting early defoaming capabilities while reducing the defoamer contact with the blood.
Blood flow from the cardiotomy reservoir to a venous bag can be intermittent or continuous depending on the frequency the suckers are used and the volume aspirated from the field. The outlet port of the cardiotomy reservoir used for adults is universally ⅜″ internal diameter as is the tube connecting the outlet of the cardiotomy reservoir to the inlet of the venous bag (see tube 153 in FIG. 1). Further, the outlet port faces straight vertically or horizontally. That combination results in the air in tube 153 to be trapped by the incoming blood above it, and dragged with that blood into the venous bag thereby adding air to the blood in the venous bag. A form of the present invention provides a fluid path between the cardiotomy reservoir and venous bag with decreased tendency to trap air.
Unpublished studies by the Inventor have shown the efficiency to remove venous air while minimizing blood volume and blood damage by blood contacting air can be expressed by a dimensionless parameter that equals the ratio of screen area available for blood flow between the venous inlet and outlet and the blood-to-air interface area. A higher ratio should result in better clinical outcome. The values for that parameter for some State-of-the-Art hard shell venous reservoirs are given in FIGS. 8c and 8d at a range of volumes. The data suggests that Terumo's hard shell venous reservoir (e.g. Capiox 18SX) should handle venous air best. Unpublished studies by the Inventor confirmed that Terumo's 18SX has the lowest bubble count at its outlet when air is added to the venous blood flowing into its inlet. FIGS. 8c and 8d also show that the values of the dimensionless parameter for the present invention (i.e. APC and VR+APC) are significantly higher than Terumo's values. For that reason, as well as others described below, the present invention handles air better than any other State-of-the-Art venous reservoir.
Hard shell venous reservoirs utilize low level detectors to either shut the arterial pump and/or actuate a tubing clamp (e.g. see Sorin's ECC.O system below) to stop outlet flow from the reservoir and prevent the reservoir from emptying and air from being pumped to the patient. A higher change in blood level per change in volume (i.e. a reservoir with smaller cross sectional area) allows for a control more sensitive to smaller changes in blood volume in the reservoir. As shown in FIGS. 8a and 8b, Terumo's hard shell venous reservoirs have the smallest cross sectional area with the highest blood level of any State-of-the-Art reservoirs. At low volumes, Terumo's reservoirs also have the largest screen area and smallest blood-to-air interface, see FIG. 8d. The present invention improves on these parameters, providing an elongated, small diameter air purging chamber with a larger screen area and smaller blood-to-gas interface per blood volume.
Numerous minicircuits that minimize operating volume and eliminate the blood-to-air interface have been introduced (e.g. Cardiovention's CORx covered by U.S. Pat. No. 6,773,670, Medtronic's Resting Heart system covered by U.S. Pat. Nos. 6,302,860, 6,524,267, Terumo's Reduced Prime Optimized Circuit also called ROCSAFE and Sorin's ExtraCorporeal Circulation Optimized, ECC.O). Each of these circuits has replaced a venous reservoir with a venous air filter to trap and remove air from the venous line and the cardiotomy reservoir. The premise of these circuits is to reduce prime volume to reduce hemodilution. Lower priming volume results in a reduction in post-bypass transfusions, reduces crystalloid fluid administration and retains plasma colloid osmotic pressure during CPB, and reduces post-CPB extravascular lung water and weight gain (edema).
Terumo's minicircuit includes an air sensor that controls both the centrifugal pump and an electronic venous occluder that allow the user to remove the air manually. Stopping blood flow to remove air may be clinically harmful, especially in cases where air enters the venous line repeatedly.
Medtronic's, CardioVention's and Cobe's minicircuits utilize a sensor that detects air in the venous line and actuates suction to remove that air and the associated electronics assure that the suction is applied only when so indicated by the sensor. Doing otherwise removes blood from the reservoir or risking air entering the outlet of the air purging chamber. A more recent invention is described by Olsen et al in US Patent Application number 20040220509 entitled “Active air removal from an extracorporeal blood circuit”. That design requires complicated electronics as displayed by FIGS. 14 through 56 of that application.
The greatest weakness of the State-of-the-Art minicircuits is their poor air removal characteristics and lack of compliance between the patient and the centrifugal (arterial) pump. Each of the active air removal systems requires a special controlling system that senses air and applies suction to remove all the air from the top of the venous filter. This requires the user to assist the upward rise and removal of air by reducing blood flows by as much as 50%. Thus, in cases where air is intermittently entrained in the venous line, the user has to reduce the blood flow to the patient repeatedly. Cobe recommends that the roller pump removing air from the venous line be set to pump 450 ml/min for 6 sec each time air is sensed in the venous line. In a worst-case scenario, 0.5 cc of air in the venous blood passing the air sensor every 6 sec (5 cc/min of air) would result in pumping over 400 ml of blood every minute out of the CPB circuit and into the cell saver or cardiotomy. Even if the air removal pump is actuated once per minute due to a small air bubble in the venous line, a patient on bypass for 90 minutes would lose 45 ml/min or, in aggregate, over 4 L. Medtronic's Active Air Removal Device and Venous Air Removal Device (VARD) can be fooled by foam that forms at the top of the venous filter, a shortfall that can result in removal of a significant volume of blood. This is especially true at high blood flow rates and may be the reason that the Medtronic's' instructions warn that blood flow should be reduced to 1.0 L/min for 30 sec at least once every hour. Further, per the “Instructions for Use”, wall suction is used to remove the air and therefore, the blood sucked with the air is also lost. Later instructions suggested that the cell saver's cardiotomy be used instead of wall suction. However, large blood volumes directed to the cardiotomy exposes that blood to the aforementioned harmful affects of the filtrate of the dirty blood, the antifoam agents, and to the large blood-air interface associated with the defoamer of the cardiotomy, all of which degrade the benefits of the minicircuits. A form of the present invention provides efficient venous air removal unmatched by any minicircuit, or State-of-the-Art hard shell or soft shell reservoirs.
The air purging chambers of State-of-the-Art minicircuits essentially placed arterial filters in the venous line. These filters' ratio of height to diameter is less than 2 and they do not communicate with ambient atmosphere. This design is not conducive to detecting small changes in volume. The present invention provides a much larger height to diameter ratio and a much larger change in height for the same change in volume, a characteristic that allows level sensors to react to much smaller volume changes.
A major objective for “venous filters” with air removal capabilities is to reduce the priming volume of the CPB circuit by removing the venous reservoir while still removing air and foam from the venous line prior to it entering the arterial pump. At least one aspect of the present invention provides further reduced volume and effectively, passively or actively, deaerates venous blood and collapses foamed blood, much like hard-shell venous reservoirs do. This action is achieved without the need to reduce pump flow or suck blood out of the circuit, and requires less contact with the defoamer while lowering foam volume (by defoaming the blood earlier), without exposing venous blood to the “dirty” blood of a cardiotomy, and with minimal or no blood loss due to air removal.
None of the minicircuits enables the user to handle clean blood except through the cell saver, which mandates a loss of plasma and platelets. The present invention deaerates clean blood before it is combined with venous blood without requiring it to pass through either a cardiotomy reservoir or a cell saver. In one form this is achieved with a two chamber cardiotomy, one chamber for the dirty blood and the other for the clean blood, with means that allow the user to start or stop blood flow from the dirty to the clean blood. Blood from the clean blood chamber is in fluid communication with the inlet of the venous reservoir.
All minicircuits utilize a centrifugal pump (CP) to draw venous blood (Kinetic Assisted Venous Drainage or KAVD) and generate arterial line pressure. KAVD has 2 major draw backs:
1. Centrifugal pumps handle air poorly; any large bubbles that pass the air-removal system and enter the centrifugal pumps are divided into much smaller bubbles that are less buoyant and are thus more difficult to remove. Large bubbles that could be trapped at the top of the arterial filter appear as smaller bubbles, able to cross the arterial filter. This problem is further exacerbated at the higher pump speeds needed when a single centrifugal pump is used both to draw venous blood and to generate the arterial line pressure.2. Centrifugal pumps maintain a fixed pressure difference between inlet and outlet. Thus, when the heart is manipulated causing the venous cannula to temporarily obstruct, venous flow stops. Flow cessation due to occlusion upstream of the pump inlet results in an inlet suction at least equivalent to the positive pressure at the centrifugal pump's outlet before the flow stopped. For example, at a flow of 5 L/min and a pump outlet pressure of +250 mmHg, an obstruction at the pump inlet would translate to the pump generating an inlet pressure of −250 mmHg or higher. A high transient suction applied at the tip of the venous cannula could suck the vessel against the cannula's open end, causing it to occlude, preventing further flow, and perhaps causing both intima and blood damage until the pump is stopped. This condition may also result in cavitation (bubble formation within the pump), a situation that could result in significant blood damage. The present invention operates in the safer VAVD mode that allows the user to set a maximum negative pressure with a vacuum regulator yet allow reverting to operating in the KAVD mode. In either the KAVD or VAVD mode of operation, the system provides better air handling than current venous reservoirs or removes air from the venous blood before it reaches the bubble dispersing centrifugal pump. Further, the presence of air at the top of the air purger of the present invention provides compliance that reduces the spikes of negative pressure inherent with current KAVD minicircuits by allowing the pump controller more time to decrease pump flow and absorbing the large pressure spikes associated with sudden stoppage of blood flow at the pump inlet. Unlike all other minicircuits, the present invention, besides providing a choice between KAVD and VAVD, also allows venous drainage by gravity. Further in gravity mode it may incorporate passive means that prevent blood from spilling over the top of the blood chamber.
When vacuum is applied to State-of-the-Art blood reservoirs then the water vapor at 37° C. of the air being purged condenses on the cooler walls of the vacuum tubing and can drip back into the blood thereby compromising sterility. Current systems eliminate that problem by inserting a sterile vapor trap between the vacuum source and the blood chamber. This increases setup time and cost. The present invention incorporates a vapor trap thereby eliminating the need for a stand alone trap.
Current vacuum assisted venous drainage capable bags (e.g. See FIG. 4 of the Instruction for Use for The V-Bag) require one connection to apply vacuum to the rigid chamber housing the bag and another to apply vacuum to the cardiotomy. A single connection to both venous bag and cardiotomy chambers would reduce setup and cost.
The aforementioned shortfalls of the State-of-Art minicircuits, venous reservoirs (whether bags or hard shell) and cardiotomy reservoirs are improved upon by the innovative designs of the present invention as more fully described below.